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Full text of "Handbook Of Chemical Engineering - I"

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General Aspects.—The term "refractories" embraces all materials used in the arts for the construction of heat-resisting containers, using the word in its broadest sense, whether it be to afford space for the evolution of gases in combustion processes, or the holding of igneous liquids or of solids undergoing calcination. The two principal functions involved in the use of refractory materials are those of thermal insulation and conduction. In a furnace refractories serve the purpose of confining the heat and preventing an excessive loss to the atmosphere; in the case of a muffle or retort it is necessary that the heat be conducted through the walls to the charge as rapidly as possible.
Although, in general, the heat-resisting quality of refractories is of paramount importance, this is by no means the only requirement and may at times be only a secondary consideration. Refractories may be expected to resist high temperatures under negligible or again under heavy loads and stresses, to resist mechanical abrasion at various temperatures, to prevent the intrusion of molten metals, slags, glass, carbon or metallic vapors, to withstand sudden temperature changes and the action of superheated steam, hydrocarbons, sulphurous oxide, chlorine and other gases. Under one set of conditions high thermal conductivity may be required and under another high insulation value, while in still other cases good electrical resistance at moderately high temperatures may be demanded. It is manifestly impossible to expect that a single refractory will fulfill all of these functions with any degree of satisfaction and hence the proper selection of a suitable material becomes an important task.
The importance of the function of refractories in industry is not generally recognized and the subject has not received the attention to which its economic significance entitles it. This is at once apparent when one considers that heat-resisting materials are indispensable in the practice of the metallurgical arts, the generation of steam, the production of coke and gas, the melting of glass, the clinkering of cement, the firing of ceramic products and innumerable other industrial processes.
Since in many reactions the yield increases rapidly with increase in temperature above a given point, it, is obvious that a gain in working temperature of only a few degrees, made possible by the use of a more refractory material, may result in so much higher returns that any additional cost of the installation is negligible.
General Properties of Refractories.—It is generally understood by practical men that the virtues and faults of refractories for a given purpose are usually determined by one or two predominating qualities possessed by a material, other considerations being of secondary importance. Not infrequently a refractory, rated low from the hcut-resisting standpoint, may be the most desirable material for some special use, say for gas producers, carburetors for blue gas, ladles for pouring steel, etc. It is desirable for this reason to consider the principal chemical and physical qualities called into play and to apply their consideration in the
* Chemist, Horner-Laughlm China Co., Newell, W. Va.